Thermally-actuated switches for protecting hermetic motor-driven compressors are classified into an exterior type and an interior type depending upon a mounting manner. The internal type thermally-actuated switch is superior to the exterior type thermally-actuated switch in a response speed to an increase in the temperature of an electric motor constituting the hermetic compressor or a refrigerant filling a hermetic housing of the compressor.
The internal type thermally-actuated switch comprises a switch body including a metal pressure-tight hermetic container and a switching mechanism provided in the container in order that characteristic changes or failure in electric conduction may be prevented even when the switch is exposed to the refrigerant or lubricant in the high-pressure hermetic housing for a long period of time. Accordingly, the switch body (hermetic container) of the thermally-actuated switch is enclosed in an electrically insulating resin case or covered with an electrically insulating covering. In either state, the thermally-actuated switch is mounted in the hermetic housing, for example, on an inner face of the hermetic housing, a hermetic terminal of the motor-driven compressor or a coil end of the electric motor.
When the thermally-actuated switch is mounted on the inner face of the hermetic housing or the hermetic terminal of the motor-driven compressor, a holder needs to be fixed to hold the thermally-actuated switch on the inner face of the hermetic housing or the hermetic terminal. Furthermore, since the inner space of the hermetic housing has recently been rendered as small as possible for the purpose of miniaturization of the compressor, limitations in mounting attitude and location of the thermally-actuated switch and holder are increased.
When the aforesaid thermally-actuated switch is mounted on the coil end of the motor, the hermetic container is covered with a cylindrical electrically insulating covering made from a heat-shrinkable polyester sheet. The thermally-actuated switch covered with the covering is fastened to the coil end with a fastening string made from polyester or the like. The aforesaid fastening string is usually the same as the one binding a coil winding.
The foregoing construction requires no specific component for mounting the thermally-actuated switch on the coil end. Furthermore, the thermally-actuated switch can be accommodated in the hermetic housing of the compressor while being integrated with the coil end.
However, there is a problem that the polyester string is slippery against the insulating coating of polyester sheet. Furthermore, there is another problem that when the cylindrical insulating coating is heat-shrunk, its part not adherent to the hermetic container hardens into indeterminate forms such that the string is difficult to cord up the thermally-actuated switch.
Furthermore, the thickness of the insulating coating is set at a small value of about 0.5 mm in view of the shrinkage characteristic and handling efficiency, so that heat is quickly transferred from the coil end to the thermally-actuated switch. However, since almost entire hermetic container is covered with the insulating covering, heat exchange is retarded between the refrigerant in the hermetic housing and the thermally-actuated switch. Furthermore, a coil with a smaller wire diameter has recently been used for miniaturization of the motor. As a result, an amount of heat generated by the motor is increased as compared with the conventional construction even when an operating current is within a normal range. Accordingly, there is a problem that the thermally-actuated switch is operated to cut off current particularly in the vicinity of an upper limit of the operating current range although the current is equal to or smaller than the operating current. There is further another problem that the thermally-actuated switch susceptibly responds to a temporary sudden increase in the temperature of the motor coil due to a short-period overload condition requiring no protecting operation.
To overcome the aforesaid problems, the applicant invented a thermally-actuated switch comprising a switch body and an electrically insulating holder holding the switch body and filed a patent application therefor (Japanese Laid-open Patent Application No. 2001-115962). The holder has a holding portion accommodating or elastically supporting the switch body. The switch body held by the holder is fastened to the coil end with a fastening string thereby to be fixed.
In the thermally-actuated switch as shown in FIG. 2 of the above-mentioned application, a large part of the surface of the switch body is exposed to the refrigerant. Consequently, heat exchange can be carried out efficiently. However, since radiant heat from the coil end is insufficiently intercepted, the thermally-actuated switch still responds to heat generated by the coil end susceptibly. There is a possibility of malfunction of the thermally-actuated switch particularly when the difference between amounts of heat generated in the normal state and under the abnormal condition is small.
Furthermore, a thermally-actuated switch as shown in FIG. 6 of the above-mentioned application comprises a switch body accommodated in a cylindrical portion. Accordingly, since a covered part of the switch body is increased, heat exchange is insufficient between the thermally-actuated switch and the refrigerant.
Therefore, an object of the present invention is to provide a thermally-actuated switch in which malfunction thereof can be prevented even when the switch is provided in a hermetic motor-driven compressor including a motor coil in which the difference between amounts of heat generated in the normal state and under the abnormal condition is small.